Asymmetric carbon atom source

As in organic chemistry, there are several sources of chirality at a metal center. As for an asymmetric carbon atom in an organic molecule, the coordination of the metal ion by four different monodentate hgands in a tetrahedral con-... 

An asymmetric carbon atom has four different atoms or groups of atoms attached to it and may be a source of dissymmetry in the molecule. The asymmetric carbon atom may have two possible arrangements of the groups around it. The two structures may be nonsuperimposable mirror images, and can be expected to differ in the rotation of plane-polarized light to an equal extent but in the opposite direction. 

A11 the molecules of sugars, amino acids, proteins will have optical isomers, because there are many asymmetric carbon atoms in their structures that have four different groups joined to a central carbon atom, but the body is very selective of the type of isomer it can use D-glucose is sweet but its L-isomer is not. The body cells like D-glucose and uses them for cell building and energy sources, but they hate L-glucose. 

The structural formula (Figure 9-23) contains three asymmetric carbon atoms, and eight different stereoisomers are possible. Only the dextrorotatory D-biotin occurs in nature and has biological activity. Biotin occurs in some products in free form (vegetables, milk, and fruits) and in other products is bound to protein (organ meats, seeds, and yeast). Good sources of the vitamin are meat, liver, kidney, milk, egg yolk, yeast, vegetables, and mushrooms (Table 9-27). 

III. Carbohydrates as Sources of Asymmetric Carbon Atoms for the Synthesis and Proof of Configuration of Biologically Important, Non-carbohydrate Compounds... 

If one analyzes the rotation of D-a-(methylenecyclopropyl)glycine (82) the optical activity must come from (at least) four sources. One rotation contribution is associated with the atomic asymmetry of the open-chain moiety (methylenecyclopropane being viewed as a ligand). On the other hand, optical activity will also be induced by the asymmetric carbon atom of the ring and the asymmetry in the electron density distribution of the exocyclic double bond system (with diastereotopic faces). Finally also helix optical activity may be operative. The example of 82 demonstrates the complexity of the optical rotation of an apparently simple cyclopropane derivative. Further discussions of optical rotations of similar compounds, therefore, will cling to only the qualitative level. 

Ancillary groups. Finally it may be noted that the various substituents of the ligand may well carry chiral centres. This is indeed a traditional way of introducing a specific chirality into a ligand. Most fi equently the source will be an asymmetric carbon atom, but there are other possibilities. The propeller-like arrangement of phenyl groups in triphenylphosphane results in two possible enantiomers, and this chirality may influence the rest of the complex. 

Use the chiral pool in synthesis and synkinesis. The common monosaccharides provide three or four asymmetrical carbon atoms per unit (an orgy in precious chiral center at very low cost). Carbohydrates together with amino acids therefore constitute the principal components of the chiral pool, the most rewarding and renewable source of fine chemicals and stereoselective membrane surfaces. 

We have already seen (in Section 3.1) that some molecules are not super-imposable on their mirror images and that these mirror images are optical isomers (stereoisomers) of each other. A chiral (asymmetric) carbon atom is the usual source of optical isomerism, as was the case with amino acids. The simplest carbohydrate that contains a chiral carbon is glyceraldehyde, which... 

Van t Hoff s first diagrams of the asymmetric tetrahedral carbon atom. Source van t Hoff, La chimie dans I espace (1875), planche I. 

For the classification of ordered or regular polymers, we shall be concerned only with stereoisomerism in the main chain. In the main chain, or, as is sometimes said, the backbone of the polymer, two kinds of stereoisomerism can exist. The first arises from asymmetric carbon atoms in the chain, and the second from double bonds that form part of the links in the backbone. The backbone is thus made up of repeating units, each of which can be a source of stereoisomerism. If the units are disposed in an ordered way we have what is called a tactic polymer. This will later be defined more specifically. 

New Aporphine Alkaloids - A number of new aporphine alkaloids have been isolated from a variety of plant sources. Structures have been assigned most often on the basis of spectra (especially nmr) and sometimes also by conversion to previously known aporphines. The stereochemistry shown below was not always proven, but is assigned when possible by the reviewers on the apparently valid assumption that all levorotatory aporphines have the D (or R) configuration at the asymmetric carbon atom and that all dextrorotatory aporphines have the L (or S) configuration at this same carbon atom. ... 

The basic ingredient of PLA is lactic acid, which is yielded from bacterial fermentation or from a petrochemical source. Lactic acid is a naturally occurring substance with the standard chemical name 2-hydroxy propionic acid. It is the simplest hydroxyl acid with an asymmetric carbon atom, and has optically active L(+) and D(—) isomers. Both L and D isomers are produced in bacterial systems, with the L isomer more commonly found. Meanwhile, mammalian systems produce only the L isomer, which is easily assimilated by enzyme protease K. Figure 4.2 shows the chemical structure of the L- and D-lactic acids. 

Flavanones are 2,3-dihydroflavones, while flavanonols are 2,3-dihydroflavonols. Indeed, flavanols are also termed dihydroflavonols and individual members may be named both from the plant source and from the derived flavonoL The most common flavanonol, for example, is known both as dihydroquercetin and as tax-ifolin. Reduction of the 2,3-double bond introduces an asymmetric carbon atom at C-2 and flavanones are capable of existing in two stereoisomeric forms. In fact, most if not all naturally occurring flavanones are laevorotatory and probably belong to the same (2S) configurational series. This has been proved rigorously for (-)-liquiritigenin (24). 

HOBACPC (p-hexyloxybenzylidene-p -amino-2-chloropropyl-cinnamate) obtained by directly attaching a chlorine atom to the asymmetric carbon closes the distance of dipole and the asymmetric carbon atom even more, resulting in doubling the spontaneous polarization value as compared to DOBAMBC [3,14,15]. 1 suspect that this is the influence from the rotational motion of the entire molecule that offsets the effect of more than one dipole. Furthermore, lactate and amino acid derivatives with directly attached chlorine atoms, bromine atoms, and cyano groups as the chiral source have also been synthesized. 

Citrate is isomerized to isocitrate by the enzyme aconitase (aconitate hydratase) the reaction occurs in two steps dehydration to r-aconitate, some of which remains bound to the enzyme and rehydration to isocitrate. Although citrate is a symmetric molecule, aconitase reacts with citrate asymmetrically, so that the two carbon atoms that are lost in subsequent reactions of the cycle are not those that were added from acetyl-CoA. This asymmetric behavior is due to channeling— transfer of the product of citrate synthase directly onto the active site of aconitase without entering free solution. This provides integration of citric acid cycle activity and the provision of citrate in the cytosol as a source of acetyl-CoA for fatty acid synthesis. The poison fluo-roacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate. 

For the optically active analogues, (+)-pulegone (782) was utilized as the chiral pool source Since its methyl substituted carbon atom is not jKrturbed during the conversion to 783 and beyond (Scheme XC), this stereocenter is fixed. Three additional asymmetric centers were then introduced as previously outlined. 

The term stereocenter (stereogenic atom) is not consistently defined. The original (Mislow) definition is given here. Some sources simply define it as a synonym for an asymmetric carbon (chiral carbon) or for a chirality center. 

Most CD spectral studies of cobalt(III) complexes have been undertaken to investigate various sources of optical activity such as distribution of chelate rings, conformation of chelate rings, vicinal effect due to asymmetric carbon in an optically active ligand, and vicinal effect due to an asymmetric donor atom. Extensive reviews on these subjects have been written by Fujita and Shimura (1), Hawkins (2), and Mason (3). 

Optically active silanes were obtained from symmetrical ketone, RCOR, in up to 46% enantiomeric excess, the catalyst being the only source of chirality. Asymmetric induction was also observed at the prochiral carbon of constitutionally unsymmetrical ketones, RCOR The optical yield at the carbon atom is different from that at silicon. This is well understood on the basis of kinetic Scheme 13. The diastereomeric complexes 56 and 57 interconvert rapidly in solution. Each complex reacts with different rates at the two faces (a and 0) of the ketone. The optical purity at the silicon center depends on the relative rates of... 

Two basic scenarios can be envisaged. The new stereogenic center is either formed within the allyl unit (type I) or at the nucleophilic carbon ( type II), respectively. However, because of the large distance between the resultant stereogenic carbon atom and the source of asymmetric induction, it is very difficult to achieve good stereocontrol, particularly for the latter process. 

Most asymmetric protonations deal with prostereogenic carbon atoms which can be enantiose-lectively protonated by chiral proton sources under strictly kinetic conditions. 

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